One type of magnetic resonance imaging (MRI) pulse sequence is referred to as balanced steady state free precession (bSSFP) MRI. bSSFP may also be referred to as TrueFISP (True Fast Imaging with Steady-state Precession). MRI involves applying radio frequency (RF) energy to an object according to carefully crafted pulse sequences in the presence of a carefully crafted magnetic field. When the object is a human body, the RF energy may be applied to tissues that are not moving (e.g., femur), tissues that are moving (e.g., heart muscle), and to the blood in the body. Since all of these parts of the body may be excited by the RF energy, all of these parts may emit nuclear magnetic resonance (NMR) signal from which an image may be reconstructed. While at times it may be desirable to acquire signal from all these parts, at other times it may be undesirable.
Flowing blood appears hyperintense in bSSFP magnetic resonance (MR) images. The hyperintense blood may be referred to as “bright blood”. The hyperintensity may be due, at least in part, to inflowing fresh magnetization, to refocusing of spins that have left an imaging slice. Bright blood may be useful at times. However, bright blood can hinder certain applications. For example, bright blood may hinder examining blood vessel walls. Bright blood can also cause artifacts in images. These artifacts may compromise image quality, may obscure underlying pathology, and so on. Thus, some conventional approaches for obtaining dark blood (DB) bSSFP images have been developed. These approaches typically lengthen the repetition time (TR) in a pulse sequence and thus lengthen overall imaging time. This can increase discomfort for a patient, reduce the number of patients that can be seen in a day, increase the likelihood that a patient will move during a scan, and so on. Thus, in general it is desirable to reduce imaging time, not to increase it. However, in some conventional approaches, TR has been lengthened to more than 11 ms. This lengthy TR both increases scan time and exacerbates banding artifacts that may be associated, for example, with accumulated phase.
Conventional TrueFISP is a coherent imaging technique. TrueFISP employs a fully balanced gradient waveform. Image contrast typically depends primarily on TR but is determined by T2*/T1 properties (or T2/T1 around TE=TR/2). T1 weighting in TrueFISP is impractical due to ever shortening TR times associated with steady state precession techniques. TrueFISP builds on FISP (fast imaging with steady state precession). FISP combines separately observed signals. But for a missing spoiler gradient pulse and RF spoiling, a FISP sequence is similar to a FLASH (fast low-angle shot) sequence. Since the spoiler pulse is missing, there may be transverse magnetization present when the next RF pulse is added to the steady state. FISP has an alternating sign RF pulse. This may be labeled in pulse sequence diagrams as α and −α (see, for example, FIG. 1). This facilitates making image contrast practically independent of T1.
Diffusion-prepared (DP) bSSFP was proposed for vessel wall imaging. See, for example, Koktzoglou I, Li D. JCMR. 2007, 9(1):33-42. The diffusion-prepared bSSFP appears to have used a unipolar gradient. Building on this work, in January of 2009, after the priority date of this application, Zhaoyang Fan, Debiao Li, et al described 3D peripheral subtraction MRA using flow-spoiled ECG-triggered balanced SSFP, in Journal of Cardiovascular Magnetic Resonance 2009, 11(Suppl 1):P288, doi:10.1186/1532-429X-11-S1-P288. In the January 2009 technique, the Koktzoglou DP module was modified by using bipolar gradient rather than unipolar gradient. The bipolar gradient was applied both before and after a central 180 degree RF pulse to address artifacts resulting from an imperfect frequency response. FIG. 11 illustrates the Fan-Li, et al modified flow-sensitizing dephasing (FSD) preparation. Note that spoiling only occurs once per FSD preparation, after the −90°x pulse.